To a hobby robotics builder, aluminum is one of the most important materials
you will use. It is very strong, light, resistant to corrosion, and affordable.
Most importantly, it is very easy to cut, shape, drill, and bend.

Strength
Aluminum is not as strong as steel, and rarely as cheap. So why is aluminum
useful? It is because aluminum has a much much higher strength to weight ratio.
This means that for a mass of aluminum and an equal mass of steel, aluminum
would be much stronger. Aluminum would be more expensive for an equal mass as
well, but would you need as much mass if it is stronger?

Corrosion Resistance and Protection Coatings
What is interesting about aluminum is that it is resistant to rust. In
actuality, aluminum does rust, but it quickly forms an oxidized layer which
acts as a protective coating against any further rust. But there are some
occasions where rust could still potentially present a problem. For example, physical wear
will 'wipe off' that protective rust layer and slowly eat away at the aluminum.
Have you ever touched aluminum that has been in water for awhile? You will notice your
fingers will turn a blackish grey when you do - thats aluminum rust.
So there are a few solutions. There are various types of aluminum such as 5052, which is a marine
grade rust resistant type. There is also this process called iriditing.
Iriditing is a cheap and fast process that basically coats aluminum in a
hard protective oxidized layer that would prevent all rust. You can either have
it specially done at some machinist shop, or buy a do it at home iriditing kit.
I have never used the home kit before, but supposedly you just paint this stuff
on with a brush and your good to go.
I have had to get this done for the robot fish I am currently working on as rust
was just icky. Plus, iriditing leaves a pretty bronzish like finish on the surface.

There are two other very common coatings to resist corrosion. Black oxidization
is about as effective as iriditing, however is a harder coat to resist physical wear
damage too. If you have ever seen a screw black in color, you probably
saw a black oxide coating. The last coating type is galvenization. This is
basically a layer of zinc placed over the metal. Corrosion will attack only the outer zinc
coating leaving the metal to be protected rust free. Its very cheap, however will quickly
get your hands dirty with contact. The ad links on the top right of this page usually
have anti-corrosion products too.

Aluminum Wear, and Anodization
Remember when you were a kid and learned that diamonds can scratch glass but glass
can't scratch diamonds? Remember how only the harder material scratched the
softer material? This is a problem for aluminum, as most metals, and even rocks,
tend to have a higher hardness. Usually this is not a big deal, unless of course
you expect other things to rub up against your aluminum often. For example, I once
had to make a cam and follower device, where a rough steel cam (rough because it
was laser cut) rubbed up against a flat follower. A steel follower would have been
ideal, except it would have been much more expensive to make, and low weight was a critical
design factor. Instead I opted to do what is called anodization. This is
somewhat expensive and difficult process involving a special bath and electricity
(that can be done by a trained machinist)
which adds an extremely hard protective layer (usually about .0004" thick) to your
aluminum. As with iriditing, anodization also changes the color. I have seen both
polished red and dull greenish-brown. Basically you can immerse the anodized
coating in a dye solution before sealing to get whatever color you want.
If you do battle bots where weight is an issue, but aluminum is too
soft for you, perhaps you can consider an anodized aluminum structure?

Thermal Abilities
Almost all heat sinks are made out of aluminum. Why? Because aluminum has a very
high thermal conductivity. Actually, I believe copper is the only other affordable and
easily available metal with a higher conductivity. But aluminum is still better because
copper is 3 times the cost, heavier, and is too soft. How is this useful for building robots?
Well this is good if you need to sink heat from your motors to your chassis. I do
not recommend making finned heat sinks on your own unless you have a CNC to
do it for you. Instead, scrap them from old computers, or buy them online such as
from Digikey.

How to Work with Aluminum.
Working with aluminum can be thickness dependent. For most robots thin sheets
well placed in your design is more than sufficient. A simple bandsaw
and drill press is usually sufficient for cutting and drilling, respectively.
Mills are nice so you can get really
clean cuts, but generally are not required unless you cut really thick
peices of aluminum. A power belt sander can clean both your rounded and straight
cuts nicely. Aluminum also taps (putting threads in a material for screws to go in to)
very easily. Sandpaper and Scotchbrite are great for rounding corners or improving
shine. Lastly, consider getting thinner aluminum sheets and bend them
into high strength custom shapes.